Gear mechanisms, such as a trapezoidal thread worm gear mechanism or a rack and pinion gear mechanism, have been used as a mechanism to convert the rotary motion of an electric motor to the axial linear motion in an electric linear actuator. These actuators are used in various kinds of driving sections. These motion converting mechanisms involve sliding contact portions. Thus power loss is increased. Accordingly, this increases the size of the electric motors which, in turn, increases power consumption. Ball screw mechanisms have been widely adopted as more efficient actuators.
In a prior art electric linear actuator, an output member connected to a nut can be axially displaced by rotationally driving a ball screw shaft, forming a ball screw, with use of an electric motor supported on a housing. Since friction of the ball screw mechanism is very low, the ball screw shaft tends to be easily rotated in a reverse direction when a thrust load is applied to the output member. Accordingly, it is necessary to hold the position of the output member when the electric motor is stopped.
An electric linear actuator has been developed where a brake mechanism is arranged for an electric motor or a low efficient mechanism, such as a worm gear, is provided as a power transmitting mechanism. In FIG. 6, one representative example is shown. It includes an actuator main body 52 with a ball screw 51 to convert the rotational motion to linear motion. A speed reduction mechanism 54 transmits the rotational motion of an electric motor 53 to the actuator main body 52 while reducing the rotational speed of the motor 53. A position holding mechanism 56 holds the position of the actuator main body 52 through its engagement with a first gear 55. The first gear 55 forms part of the speed reduction mechanism 54.
The ball screw 51 includes a screw shaft 57 that acts as an output shaft. It is formed with a helical screw groove 57a on its outer circumference. A nut 58 is fit on the screw shaft 57. The nut 58 is formed with a helical screw groove 58a on its inner circumference. A number of balls 59 are rollably contained in a rolling path formed by the opposite screw grooves 57a, 58a. 
The actuator main body 52 includes the nut 58 rotationally supported on the inner circumference of a housing 60 by a pair of ball bearings 61, 62. The screw shaft 57 is axially movably supported but not rotationally relative to the housing 60. Thus, the rotational motion of the nut 58 driven by the speed reduction mechanism 54 can be converted to the linear motion of the screw shaft 57.
The speed reduction mechanism 54 includes the first gear 55 formed as a small spur gear fit on a motor shaft 53a of the electric motor 53. A second gear 63 mates with the first gear 55. The second gear 63 is integrally formed with the nut 58 as a large spur gear.
The position holding mechanism 56 includes a shaft 64 that functions as a locking member. The shaft 64 is adapted to engage with the first gear 55. A solenoid 65, functioning as a driving mechanism, engages and disengages the shaft 64 with the first gear 55. The shaft 64 has a rod-like configuration and is linearly driven by the solenoid 65. Thus, its tip end can be received in a recess 66. Since rotation of the first gear 55 can be prevented by the shaft 64, due to engagement of the shaft 64 with the first gear 55 by controlling the solenoid 65, it is possible to stably hold the position of the screw shaft 57 of the actuator main body 52. See, Patent Document 1: JP 2009-156416 A.
In the prior art electric linear actuator 50, since the rotation of first gear 55 can be firmly prevented by the shaft 64, due to the engagement of the shaft 64 with the first gear 55 by controlling the solenoid 65, it is possible to stably hold the position of the screw shaft 57 of the actuator main body 52 without causing any slippage between engaging surfaces.
However, it is believe that the control of the electric linear actuator 50 would be impossible in a case of power deficiency due to voltage drop of a battery. Under the circumstances, the nut 58 would be reversely rotated and continue the reverse rotation due to its inertia moment when a pushing thrust load is applied to the screw shaft 57. As the result, it is believed that the tip end of the screw shaft 57 would abut against an inner wall surface of the housing 60. Thus, this causes a lock up operation that would disable return of the screw shaft 57 by the electric motor 53 itself.
In addition, the nut 58 is urged toward the right-hand direction by a reaction force, inverse thrust load, applied to the nut 58 by the screw shaft 57 when the tip end of the screw shaft 57 abuts against an inner wall of the housing 60. Thus, the ball bearing 62 of the paired ball bearings 61, 62 would also be urged against the opposite inner wall of the housing 60. Accordingly, this sometimes causes a locking up operation of the linear actuator 50.
Furthermore, in the speed reduction mechanism 54, with a single-gear combination of the first gear 55, a spur gear of small diameter, and a second gear 63, a spur gear of large diameter, sometimes a two-stage reduction mechanism has been adopted. This occurs by providing an intermediate shaft between the motor shaft 53a and the screw shaft 57 when a larger reduction ratio is required or when there is any layout limitation. In this case, problems exist such as increasing the cost in machining and increasing the of number and weight of parts. Accordingly, an intermediate shaft structure is desired that has excellent rotational performance and can suppress the above problems to a minimum.